Handover is the basic means of providing mobility in cellular network architectures. Handover occurs when a call has to be handed off from one cell to another, as the user moves between cells. For a user having an ongoing communication and crossing a cell edge, it is necessary to use radio resources in the “new” cell, also called the target cell, because the signal strength from the “old” cell deteriorates as the user moves closer to the target cell. The whole process of tearing down an existing connection in the old or current cell and establishing a new connection in the target cell may be referred to as handover. Handover between base stations from the same access network technology is well defined. However, when handover is taking place between different technologies, it may take several seconds to register with the target network. In a particular example, when a mobile device moves from a 4th Generation (4G) or Long Term Evolution (LTE) network to an Evolution-Data Optimized (EVDO) network, the registration to EVDO can take up to five seconds. This long handover delay can cause a significant interruption in service, resulting in poor user experiences. Since mobile devices are typically required to register with a base station of any new cell, delays associated with registration can be a frequent occurrence for mobile device users roaming between networks.
There are generally two types of handover, hard handover and soft handover. In a traditional “hard” handover, the connection to the current cell is broken, and then the connection to the new cell is made. This is known as a “break-before-make” handoff. Hard handovers are used mainly in Global System for Mobile communications (GSM) cellular systems, where each cell was assigned a different frequency band. In such systems, a user entering a new cell tears down the existing connection before setting up a new connection at a different frequency in the target cell. The mobile terminal performs a handover, when a signal strength of a neighboring cell exceeds a signal strength of the current cell within a given threshold. In a typical hard handover, users may experience a “click” sound when speech transmissions are stopped momentarily during handovers.
Code division multiple access (CDMA) cellular systems use identical frequencies in each cell. Hence, it is possible to make a connection with a new cell before leaving an old cell. This technique is known as a “make-before-break” or “soft” handover, where a mobile terminal in proximity to a cell boundary can communicate with two or more base stations. Soft handover has been used in CDMA networks, and later in 3G or Universal Mobile Telecommunications System (UMTS) networks. Soft handover maintains two thresholds for its handover decision making. When a pilot signal strength from a base station exceeds a first threshold, the base station (or an identifier associated with the base station) is put into the candidate set. The candidate set may contain more than one base station (or base station identifier) at any time. Depending on changes in the pilot signal strength from the two or more base stations involved, a hard decision will eventually made to communicate with only one base station. This will occur when the signal from one base station is considerably stronger than signals from other base stations, or the signal from one base station exceeds a second threshold. In an interim period (e.g., between two thresholds), the user may have simultaneous communication with all base stations in the candidate set.
Compared to hard handover, soft handover is more complex, requiring additional network resources. On the other hand, soft handover offers the advantage of smoother communications, without a “click” sound typically found in hard handover during speech communications. Soft handover also reduces a “ping-pong” effect of a pilot signal that is continuously added and deleted, which is common in hard handovers. Reducing or eliminating the ping-pong effect reduces traffic load on networks by limiting handover signaling and overhead.